1. Field of the Invention
The present invention relates to the field of the catalysis of dehydrogenative condensation reactions enabling the polymerization/crosslinking of silicone. The reactive species involved are monomers, oligomers, and/or polymers of polyorganosiloxane nature.
2. Description of Related Art
The reactive units concerned in these species are ≡SiH units and ≡SiOH units. Dehydrogenative condensation between these reactive silicone units results in the formation of ≡Si—O—Si≡ bonds and in the release of hydrogen gas.
This dehydrogenative condensation is an alternative to the polymerization/crosslinking routes known in the silicone field, namely the polyaddition route by reaction between ≡SiH and ≡Si-alkenyl(vinyl) units, and also the polycondensation route by reaction between ≡SiOR and ≡SiOR units (with R=alkyl). All these polymerization/crosslinking routes result in silicone products which are more or less polymerized and more or less crosslinked, which can constitute products that can be used in many applications: adhesives, sealing products, pointing products, adhesion primers, release coatings, foams, etc.
It is known according to French patent FR-B-1 209 131 that a reaction between a silanol Ph2Si(OH)2 and a said organosiloxane [(Me2HSi)2O] with Me=methyl and Ph=phenyl, by means of dehydrogenative condensation, can be catalyzed by a chloroplatinic acid (H2PtCl6.6H2O).
It is also known practice to use a rhodium complex (RhCl3[(C8H17)2S]3), for example as mentioned in U.S. Pat. No. 4,262,107, a platinum complex such as the Karstedt catalyst, or metal catalysts based on platinum, rhodium, palladium or iridium. As iridium-based catalyst, mention may be made of the following compounds: IrCl(CO)(TPP)2, Ir(CO)2 (acac), IrH(Cl)2(TPP)3, [IrCl(Cyclooctene)2]2, IrI(CO)(TPP)2 and IrH(CO)(TPP)3, in which formulae TPP signifies a triphenylphosphine group and acac an acetylacetonate group.
Other examples are catalysts such as amines, colloidal nickel or dibutyltin dilaurate (see the book by Noll, “Chemistry and technology of silicones”, page 205, Academic Press, 1968-2nd edition). However, the alkyltin-based catalysts, although very effective, usually colorless, liquid and soluble in silicone oils, have the drawback of being toxic (CMR2 toxic for reproduction).
Other catalysts, such as boron derivatives of tris(pentafluorophenyl)borane type, are described in French patent application FR-A-2 806 930.
U.S. Pat. No. 4,262,107 describes a silicone composition comprising a polydimethyldisiloxane comprising silanol ends, a crosslinking agent consisting of a polyorganosiloxane comprising ≡SiH units in the chain and comprising trimethylsilyl ends and a catalyst consisting of a rhodium complex (RhCl3[(C8H17)2S]3). This silicone composition that can be crosslinked by means of dehydrogenative condensation in the presence of a rhodium complex can be used for producing release coatings on flexible supports such as paper and plastic or metal films. The crosslinking is carried out at a temperature of 150° C.
European patent application EP-A-1 167 424 describes the obtaining of linear silicone block copolymers by means of the dehydrogenative condensation of a polydimethylsiloxane comprising silanol ends and of a polyorganosiloxane comprising aromatic groups and comprising ≡SiH ends in the presence of a metal catalyst which may be based on platinum, rhodium, palladium or iridium, platinum being particularly preferred.
French patent application FR-A-2 806 930 describes the use of boron derivatives of tris(pentafluorophenyl)borane type as a thermoactivatable catalyst for dehydrogenative condensation between a polyorganosiloxane comprising ≡SiH units and a polyorganosiloxane comprising ≡SiOH end units. Such silicone compositions that can be crosslinked by means of dehydrogenative condensation in the presence of Lewis acids of boron derivative type can be used for the manufacture of release coatings on flexible supports, in particular on paper, and also in the manufacture of crosslinked silicone foams in which the release of hydrogen and the quality of the crosslinking network are controlled.
It emerges from this review of the prior art, relating to the catalysis of dehydrogenative condensation between a polyorganosiloxane comprising a siloxyl ≡SiOH unit and a polyorganosiloxane comprising a siloxyl ≡SiH unit, that there is a significant need to:
1) find new nontoxic catalysts,
2) reduce the catalyst activation temperature, and
3) limit the side reactions.
Application FR 03 41232 relates to a process for preparing a polyorganosiloxane by polymerization via ring opening and/or redistribution of polyorganosiloxane, in the presence of a catalyst (or initiator) consisting of at least one nucleophilic carbene.
Application FR 06 03563 relates to a process for polycondensation of a polyorganosiloxane between at least one ≡SiOH unit and at least one ≡SiOR unit also using catalysts with a carbene structure.
However, the carbene catalysts described in the abovementioned two applications are unstable and must generally be either used in a crystalline form, or generated in situ from precursor salts. As it happens, carbenes are highly reactive species that degrade in the open air, which implies storing them and handing them under an inert atmosphere.